Field of the Invention
The present invention relates to a circuit configuration for driving a pulse width modulator in a switched mode power supply having a difference formation stage for forming a difference signal from a control signal, which can be supplied to a first input terminal, and a reference signal, which can be supplied to a second input terminal, and for producing a drive signal, which can be tapped off at an output terminal.
A pulse width modulator in a switched mode power supply produces drive pulses for a semiconductor power switch connected in series with the primary coil of a transformer. The frequency and duration of the drive pulses are determined by the power which is consumed by the primary coil, and hence by the switched mode power supply, and which, in turn, is output to a variable load connected to the secondary side of the transformer. During operation, the voltage drop across the load is intended to be kept constant as independently of the load as possible. This requires a reduction in the power consumption, and hence in the frequency and/or duration of the drive pulses, as the load is reduced, and an increase in the power consumption, and hence in the frequency and/or duration of the drive pulses, as the load is increased.
The output signal delivered to the pulse width modulator from the difference formation stage is used to set the duration and/or frequency of the drive pulses delivered by the pulse width modulator. In the case of switched mode power supplies having a fixed clock, the drive pulses in the pulse width modulator are generated through the use of a sawtooth generator, for example, the drive pulses starting and ending with rising edges of the generated sawtooth signal when the amplitude of the sawtooth signal exceeds the amplitude of the output signal delivered by the difference formation stage.
Output voltage fluctuations caused by load changes are returned by the control signal, which is dependent on the output voltage, to the difference formation stage, in which the control signal is compared with the reference signal and the drive signal is readjusted until the desired output voltage is established or a minimum possible difference is established between the control signal and the reference signal.
In the simplest case in known switched mode power supplies, the difference formation stage, acting as a controller, includes a differential amplifier for forming a difference signal from the control signal and the reference signal, with the difference signal subsequently being amplified by a constant gain factor. As a result of additional circuit measures, the differential amplifier, which has its own proportional control response, can be used to form a controller having an integral or a proportional/integral control response.
Irrespective of whether the difference formation stage is in the form of a proportional controller, an integral controller or a proportional/integral controller, the known switched mode power supplies have stability problems if the load which is to be driven, and hence the power consumption, is very small, as is the case for hi-fi or television equipment in standby mode, for example. In theory, in this case, short turn-on pulses at relatively long time intervals would be sufficient for the power consumption, in order to keep the voltage across the small load constant. On the other hand, the pauses between individual drive pulses must not exceed 50 .mu.s, or the frequency of the drive pulses must not fall below 20 kHz, since this would cause the switched mode power supply to hum audibly. When the drive pulses have a preset frequency of higher than or equal to 20 kHz, the drive pulses for very small loads are indeed so short that the semiconductor power switch, which is usually in the form of a power FET, is no longer on, but rather changes to the active range. In the active range, small changes over the duration of the drive pulses cause large changes in the drain voltage of the power FET, and hence large changes in the load current flowing through the power FET and in the power consumed by the switched mode power supply. This results in instabilities in the readjustment of the drive signal generated by the difference formation stage. Limit cycling results, i.e. the power FET, as a result of too small a drive signal, is not turned on at all for a time and is then turned on fully again for a number of correspondingly longer drive pulses; the switched mode power supply is in burst mode. This results in undesirable acoustically audible noises from the switched mode power supply.
To date, this problem has been solved either in a complex and expensive manner by acoustic damping of the winding parts or by provision of a minimum load, which necessitates a power consumption at which the switched mode power supply does not change to burst mode. U.S. Pat. No. 4,007,361 discloses a prior art circuit configuration. This circuit configuration includes a difference formation stage for forming a difference signal from a control signal and a reference signal and for producing a drive signal, which can be tapped off at an output terminal, by amplifying the difference signal by a gain factor which is dependent on the drive signal and the control signal. The output of the difference formation stage has a multiplier to which the amplified difference signal and the drive signal fed back via a feedback network are supplied as input signals. Furthermore, U.S. Pat. No. 5,594,631 describes a circuit configuration for driving a pulse width modulator, the circuit configuration having a difference formation stage to which a control signal and a reference signal are supplied. The difference formation stage produces the difference signal between the control signal and the reference signal and supplies the difference signal to an amplifier whose gain is controlled on the basis of the control signal.